Method and installation for remineralizing raw water

ABSTRACT

The invention relates to a method and installation for treating water for human consumption, for an industrial process, for agricultural purposes or for other purposes. The method used to remineralize raw water comprises a stage in which carbon dioxide and at least one inorganic base are injected. According to the invention, the method comprises the following stages: the flow Q of raw water is separated into a flow Q 1  and a flow Q 2;  only flow Q 1  is remineralized by injecting carbon dioxide and at least one inorganic base, said remineralization being carried out in at least one remineralization basin; remineralized flow Q 1  is mixed with untreated flow Q 2.

[0001] This invention relates to the field of water treatment. Moreprecisely, the invention relates to a method and installation fortreating water intended for human consumption, for an industrialprocess, for agricultural use or for other uses.

[0002] In this field, it is often necessary to carry out a treatment tocorrect the aggressivity of the water or to remineralize it, inparticular when said water has a pH ranging between. 2 and 9, lowalkalinity and a low concentration of calcium and/or magnesium.

[0003] A treatment such as this can be carried out at the beginning ofthe line (pre-remineralization), along the line (inter-remineralization)or at the end of the line (final remineralization). The treatment maythus require one or more of these steps depending on the quality of theraw water.

[0004] Now, failure to remineralize the water may result in either anaggressivity of the water with respect to calcium, concrete or cement,or a corrosivity of the water with respect to metals. The structures andequipment associated with the water system may therefore be damaged, oreven destroyed, and the water becomes turbid and acquires a red, greenor other type of coloring.

[0005] In order to avoid such consequences, a partial or completeremineralization of the raw water is normally carried out in order tomake it less aggressive or even non-aggressive with respect to all ofthe concrete containers (tanks, pipes, pipelines, basins, etc. . . . )and to make it likewise less corrosive with respect to metals.

[0006] The degree of calcium carbonate equilibrium of water is generallymeasured by its saturation index, which is defined as being thedifference between the pH of the water and the pH that it ought to haveat the same degree of alkalinity in order to be in equilibrium(saturation pH).

[0007] Although the quality of raw water can vary greatly, it often hassaturation index values ranging between −2 and −3.

[0008] By carrying out a partial remineralization, the saturation indexis successfully brought to values ranging between −0.3 and −0.5, whereasafter total remineralization, the water is brought to calcium carbonateequilibrium (saturation index equal to 0).

[0009] Therefore, the purpose of this type of treatment is to bring thesaturation index to a value of 0 or close to 0.

[0010] Traditionally, this operation is carried out by injecting carbondioxide and an inorganic base such as lime, soda, sodium carbonate orsodium bicarbonate. The mixing together of the reagents is carried outby means of a static mixer or in separate tanks (injection of onereagent per tank), or else in a single tank in which the two reagentsare injected at the same time.

[0011] According to known techniques, the remineralization operation iscarried out on the entire flow of water being treated, as illustrated inFIG. 1.

[0012] This technique has the disadvantage of requiring heavy equipmentincluding, in particular, large numbers of tanks. As a matter of fact,experience shows that the costs generated by the required equipmentsubstantially increase the overall cost of the water treatmentinstallation, this being proven true in particular for largeinstallations which require heavy maintenance and operation.

[0013] In particular, the invention has the purpose of alleviating thisdisadvantage of the state of the art.

[0014] More precisely, one purpose of the invention is to propose amethod for remineralizing raw water which requires structures andequipment that are less bulky than those used with the prior arttechniques.

[0015] The invention also has the purpose of proposing such a methodwhich makes it possible to obtain substantial savings with respect tothe costs associated with the installation, operation and maintenance ofthe prior art solutions.

[0016] These purposes, as well as others which will become apparenthereinafter, are achieved by means of a method for remineralizing rawwater of the type including a step of injecting carbon dioxide and atleast one inorganic base. According to the invention, the methodcomprises the steps of

[0017] separating the flow Q of raw water into a flow Q1 and a flow Q2;

[0018] remineralizing only said flow Q1 by injecting carbon dioxide andat least one inorganic base, the remineralization being carried out inat least one remineralization tank;

[0019] mixing the remineralized flow Q1 with said untreated flow Q2.

[0020] A remineralization method is thus obtained whose maintenance andoperation are simplified, while at the same time making it possible toobtain considerable savings on the cost of the equipment and accessories(pumps, valves, check valves, tubes, etc. . . . ).

[0021] In addition, the comparison between a remineralization of theentire flow and of only one portion of the flow according to theinvention reveals large savings in space and work surfaces.

[0022] It shall be noted that a technique was known in prior art,according to EP 520 826, wherein a by-pass is used for injecting thereagents being used in the remineralization step. However, the actualremineralization itself takes place in a tank receiving the entire flowof water being treated and not, as in this invention, only a portionthereof.

[0023] According to a first advantageous solution, said inorganic basecomprises lime injected in the form of lime milk.

[0024] According to another advantageous solution, said inorganic basecomprises lime injected in the form of lime water. In this case, saidinjection of lime water is carried out by means of a lime saturator.

[0025] A rather precise adjustment in the desired final alkalinity isthus obtained by means of the lime saturator.

[0026] Other inorganic bases may of course be used in other embodimentswithout exceeding the scope of the invention.

[0027] According to one preferred solution, said injection of carbondioxide is carried out either from the vents of a degassing tower forwater charged with carbon dioxide or with the pure product.

[0028] Said flow Q₁ ranges advantageously between 5 and 95% of the flowQ.

[0029] By proceeding in this way, with a remineralization of only aportion of the raw water, the desired final qualities are successfullyobtained in terms of alkalinity and calcium concentration, thecorresponding saturation index falling between −0.5 and +0.1.

[0030] According to one particularly advantageous solution, said flow Q₁is less than said flow Q₂.

[0031] In the case where the flow treated by an addition of reagents isless than 50% of the flow of raw water, substantial economic savings areactually obtained.

[0032] Preferably, said flow Q₁ represents approximately 30% of flow Q.

[0033] In this particular case, it shall be noted that it is possible touse structures that are three times less bulky than those used with theconventional complete remineralization techniques. Of course, otherembodiments modifying the fraction of the treated flow may beanticipated without exceeding the scope of the invention.

[0034] According to one advantageous variant, the method includes atleast one additional treatment step (clarification, oxidation,polishing) upstream and/or downstream from said treatment step of saidflow Q₁.

[0035] Depending on the quality of the raw water and whether a partialor complete remineralization is desired, a final injection of thereagents (lime, soda, sodium carbonate) may actually be carried out overthe entire flow after mixing the partial flows Q₁ and Q₂.

[0036] The invention also concerns any installation for theremineralization of water comprising at least one main water supplypipeline and means of injecting carbon dioxide and at least oneinorganic base, characterized in that it includes at least one by-passpipeline for a portion of the flow coming from the main pipeline, atleast one remineralization tank provided on said by-pass pipeline, saidinjection means being provided on said by-pass pipeline and/or on saidremineralization tank.

[0037] Other characteristics and advantages of the invention will becomemore apparent upon reading the following description of one preferredembodiment of the invention given as a non-limiting, illustrativeexample, and of the appended drawings in which:

[0038]FIG. 1 is a schematic illustration of a remineralization methodaccording to prior art

[0039]FIG. 2 is a schematic illustration of a remineralizationinstallation according to the invention wherein only a fraction of theflow of raw water is treated.

[0040] As illustrated schematically in FIG. 1, the remineralizationmethod according to the state of the art treats the entire flow Q ofwater to be remineralized by injecting carbon dioxide and lime (oranother reagent).

[0041] According to the invention, as shown in FIG. 2, the flow Q ofwater to be remineralized is separated into two flows Q₁ and Q₂.

[0042] Flow Q₁, corresponding, according to this embodiment, to 30% offlow Q, undergoes a thorough remineralization treatment by injectingcarbon dioxide and by adding appropriate reagents, in this case limewater, the injection of the latter being carried out by means of a limesaturator.

[0043] Another embodiment might consist in injecting the lime in theform of lime milk.

[0044] The injection of carbon dioxide is carried out from the vents ofa degassing tower (not shown) for water charged with carbon dioxide. Inother embodiments, the injection of carbon dioxide may be carried outusing the pure product.

[0045] According to FIG. 2, the embodiment of an installation for theremineralization of water according to this invention shows a main watersupply pipeline 1 and means of injecting 2, 3 CO2 and lime. Thisinstallation also has a by-pass pipeline 4 for a portion of the flow ofwater coming from the main pipeline 1, and a remineralization tank 5provided on said by-pass pipeline 4. According to this invention, theinjection means 2, 3 are provided on the remineralization tank 5. Theportion of the flow of water remineralized in the tank 5 is thenreturned to the main pipeline 1. The installation also includes means ofinjecting 6 lime into the main pipeline 1 after the portion of the flowremineralized in the tank 4 has been returned to said pipeline.

[0046] As shown clearly in FIG. 2, the flow Q₂ forms a by-pass of thetreatment step undergone by the flow Q₁.

[0047] Upon exiting the treatment step of flow Q₁, the flows Q₁ and Q₂are mixed together, thus forming a flow Q of remineralized water.

[0048] According to this preferred embodiment of the invention, the flowQ of remineralized water may undergo an additional treatment step byadding lime, depending on the final desired water quality.

[0049] The advantages of the method according to the invention will nowbe highlighted by a comparative study of a complete remineralizationmethod according to the state of the art and a method of remineralizingonly a fraction of the flow of raw water according to the invention.

[0050] This comparative study is conducted using raw water having thefollowing characteristics:

[0051] flow rate: 1,000 m³/h;

[0052] temperature: 20° C.;

[0053] pH: 6.5;

[0054] free CO₂: 6.72;

[0055] alkalinity: 10 mg/l CaCO₃;

[0056] calcium: 2 mg/l.

[0057] After treatment within the framework of a method forremineralizing the entire flow of raw water, the latter has thefollowing characteristics

[0058] flow rate: 1,000 m³/h;

[0059] pH: 7.87;

[0060] saturation pH: 7.87;

[0061] saturation index: 0;

[0062] free CO₂: 2.71 mg/l.

[0063] alkalinity: 104 mg/l CaCO₃;

[0064] calcium: 39.9 mg/l.

[0065] During the course of the method for remineralizing the entireflow of raw water, the carbon dioxide is injected at the rate of 76 g/m³and the lime at 69.56 g/m³, the lime saturator required for implementingthis method having the following characteristics:

[0066] mirror velocity: 6 m/h;

[0067] unit area: 1,000/6=166.6 m²;

[0068] volume of the reaction tank: 1,000×10/60=166.6 m³ (for a contacttime of 10 min.).

[0069] Within the framework now of the method according to theinvention, during the course of which only 30% of the flow of water tobe remineralized is treated, the treatment is carried out by injectingcarbon dioxide at the rate of 222 g/m³. The fraction of the flow treatedhas the following characteristics

[0070] flow rate: 300 m³/h;

[0071] pH: 7.06;

[0072] saturation pH: 7.06;

[0073] saturation index: 0;

[0074] free CO₂: 42.73 mg/l;

[0075] alkalinity: 270 mg/l CaCO₃;

[0076] calcium: 106 mg/l.

[0077] The saturator used to obtain this result has the followingcharacteristics:

[0078] mirror velocity: 6 m/h;

[0079] unit area: 300/6=6=50 m²;

[0080] volume of the reaction tank: 300×10/60=50 m³ (for a contact timeof 10 min.).

[0081] After mixing the flow of treated water and the flow of untreatedwater, the water has the following characteristics:

[0082] flow rate: 1,000 m³/h;

[0083] pH: 6.99;

[0084] saturation pH: 8.01;

[0085] saturation index: −1.02;

[0086] free CO₂: 17.52 mg/l;

[0087] alkalinity: 88 mg/l CaCO₃;

[0088] calcium: 33.2 mg/l.

[0089] After a final step consisting in adding lime at the rate of 12.4g/m³, the water has the following characteristics at the end of themethod according to the invention:

[0090] flow rate: 1,000 m³/h;

[0091] pH: 7.86;

[0092] saturation pH: 7.86;

[0093] saturation index: 0;

[0094] free CO₂: 2.77 mg/l;

[0095] alkalinity: 104.8 mg/l CaCO₃;

[0096] calcium: 39.92 mg/l.

[0097] It is noted then, that for a remineralized water quality havingalmost identical characteristics after being treated, the lime saturatorused within the framework of the method according to the inventionrequires a unit area more that three times smaller-that that of the limesaturator used within the framework of the method for remineralizing theentire flow of raw water.

[0098] Therefore, this comparative study demonstrates that the methodaccording to the invention makes it possible to obtain substantialsavings with respect to the total investments (equipment andaccessories) associated with a water treatment installation.

[0099] Other embodiments using the principle of the method justdescribed for remineralizing only a fraction of the flow of raw watermay, of course, be anticipated without exceeding the scope of theinvention.

1. Process for remineralizing raw water of the type including a step ofinjecting carbon dioxide and at least one inorganic base, characterizedin that it comprises the steps of: separating the flow Q of raw waterinto a flow Q1 and a flow Q2; remineralizing only said flow Q1 byinjecting carbon dioxide and at least one inorganic base, theremineralization being carried out in at least one remineralization tankmixing the remineralized flow Q1 with said untreated flow Q2.
 2. Methodfor remineralizing raw water as claimed in claim 1, characterized inthat said inorganic base is selected from the following bases: lime,soda, sodium carbonate, sodium bicarbonate.
 3. Method for remineralizingraw water as claimed in claim 2, characterized in that said inorganicbase comprises lime injected in the form of lime milk.
 4. Method forremineralizing raw water as claimed in claim 2, characterized in thatsaid inorganic base comprises lime injected in the form of lime water.5. Method for remineralizing raw water as claimed in claim 4,characterized in that said injection of lime water is carried out bymeans of a lime saturator.
 6. Method for remineralizing raw water asclaimed in any of claims 1 to 5, characterized in that said injection ofcarbon dioxide is carried out either from the vents of a degassing towerfor water charged with carbon dioxide or with the pure product. 7.Method for remineralizing raw water as claimed in any of claims 1 to 6,characterized in that said flow Q₁ ranges between 5 and 95% of the flowQ.
 8. Method for remineralizing raw water as claimed in claim 7,characterized in that said flow Q₁ is less than said flow Q₂.
 9. Methodfor remineralizing raw water as claimed in claim 8, characterized inthat said flow Q₁ represents approximately 30% of flow Q.
 10. Method forremineralizing raw water as claimed in any of claims 1 to 9,characterized in that it includes at least one additional treatment stepof clarification, oxidation, polishing, upstream and/or downstream fromsaid treatment step of said flow Q₁.
 11. Installation for theremineralization of water comprising at least one main water supplypipeline (1) and means of injecting carbon dioxide and at least oneinorganic base (2, 3), characterized in that it includes at least oneby-pass pipeline (4) for a portion of the flow coming from the mainpipeline (1), at least one remineralization tank (5) provided on saidby-pass pipeline (4), said injection means (2, 3) being provided on saidby-pass pipeline (4) and/or on said remineralization tank (5).